1. Field of the Invention
This invention relates generally to location of emitters of electromagnetic radiation, and in particular, to Time Difference of Arrival (“TDOA”)-based location of emitters of electromagnetic radiation.
2. Description of the Related Art
TDOA techniques have been employed in the past to determine the location of emitters of electromagnetic radiation, such as radio frequency (“RF”) emissions. Using the TDOA technique, the time difference (TDOA) in reception of a radio signal received at multiple spaced sensing platforms may be used to define a hyperboloid upon which the RF emitter of interest is located. TDOA may be used to calculate the geolocation of the RF transmitter if three or more sensing platforms simultaneously capture the radio signal, and if the difference in the times of arrival of the captured radio signal at the different sensing platforms can be accurately determined.
Frequency difference (FDOA) in reception of the same radio signal at two spaced apart moving sensing platforms (such as aircraft) may be generated by moving the two spaced apart sensing platforms at different velocities and headings relative to an RF emitter, and may be used to further resolve the geolocation of the RF emitter on a hyperboloid defined by the TDOA in reception of a radio signal received at the sensing platforms of the two sensing platforms. In this regard, assuming that the RF emitter of interest is located on the surface of the earth, the intersection of the TDOA hyperboloid, the FDOA surface, and the surface of the earth may be employed to determine a set of possible geolocations for the RF emitter. However, only one of these possible geolocations is real, the other possible geolocations in the set are purely mathematical solutions that are not the real location of the radio emitter. To determine the correct geolocation requires additional information. This additional information may be obtained from a system that can produce a line of bearing, from a third aircraft that can measure an independent TDOA/FDOA set of data, or by repositioning the two aircraft and then measuring a second TDOA/FDOA set of data.
TDOA/TDOA and FDOA/FDOA techniques have also been employed in the past to determine the geolocation of emitters of electromagnetic radiation, such as radio frequency (“RF”) emissions. For example, using a TDOA/TDOA technique, the time difference (TDOA) in reception of a radio signal received at three spaced apart sensing platforms may be used to define two hyperboloids upon which the RF emitter of interest is located. The intersection of the two TDOA hyperboloids and the surface of the earth may be employed to determine a set of possible geolocations for the RF emitter, which may be further resolved as previously described above. In another example, using a FDOA/FDOA technique, frequency difference (FDOA) in reception of the same radio signal at the three spaced apart sensing platforms may be employed to generate two FDOA curves, and the intersection of the two TDOA hyperboloids and the surface of the earth may be employed to determine a set of possible geolocations for the RF emitter, which also may be further resolved as previously described above.
In a conventional TDOA-based geolocation system architecture, a coordinated timing reference between the sensing platforms must be established so that the difference in the times of arrival of a captured radio signal at the different sensing platforms can be accurately determined. Conventional geolocation methods have used a precision clock source to establish a coordinated timing reference for the different sensing platforms. One type of such a precision clock source is a disciplined rubidium oscillator (DRO) which is slaved to a common clock source such as the one pulse per second (pps) clock provided by a GPS receiver.